The present invention relates to a method of lining a fluid conduit, apparatus for use in such a method, and a carrier which is displaceable within a fluid conduit for servicing purposes therein. The invention was primarily developed for lining, with a tubular liner, a main fluid conduit (such as a sewer, water, gas or other fluid pipe) having a branch conduit or pipe extending therefrom. The tubular liner will usually be a plastics pipe which is pulled through the main fluid conduit. With the liner positioned in the main fluid conduit it is necessary to form ports in the liner by cutting through the wall thereof at positions which coincide with respective branch conduits or pipes so that fluid flow is permissible between the branch conduit and the bore of the liner. The cutting of a port in the liner may be achieved by a water jet cutting device mounted on a carrier within the liner in accordance with the disclosure in our British Pat. No. 2,098,300B. Prior to the ports being cut, it is of course necessary to locate, with reasonable accuracy, the respective openings from the main fluid conduit to the branch conduits or pipes so that when the ports are cut they will substantially coincide with their respective branch conduits. With relatively thick walled and substantially rigid plastics liners it will be appreciated that location of an appropriate position for a port visually (for example, by use of a television camera mounted on a carrier within the liner) cannot be relied on. As a consequence, prior proposals to locate the position at which a port should be cut in the liner for a buried sewer pipe include excavation techniques where a branch conduit is broken open to mark the wall of the liner through the opening of the branch conduit so that such marking is detectable from the interior of the liner and subsequently repairing the branch conduit. A further proposal is to position a radioactive source in the branch conduit, either by excavating and breaking through the wall of the branch conduit or by maneuvering the source through the branch conduit from the end thereof remote from the main conduit and then detecting the position of the radioactive source and thereby the branch conduit from within the liner. It will be appreciated that the prior proposals for determining the port locations in the liner can prove expensive, lengthy and often inconvenient and it is an object of the present invention to alleviate the disadvantages associated with such prior proposals. It is also an object of the present invention to provide an apparatus which can be used to determine, with reasonable accuracy, the location at which a port should be cut within a liner. A further object is to provide a carrier which can be used within a fluid conduit to facilitate positioning of a particular service on that carrier.